This invention relates to processes and apparatus for sugar extraction from cane, and more particularly to an efficient diffusion extraction process.
In present day sugar diffusion processes, the entire cane is usually subjected first to a cutting or shredding process by which the cane sugar cells in the pith and rind are ruptured open, or at least in which a substantial number of such cells are ruptured. The finely divided cane is then subjected to diffusion, where the refined pulp material is applied to one of a number of different diffusers at a relatively high consistency, such as about 8%-12% solids or higher. Heated water or steam is applied and juices and liquids are extracted in a tank or on a conveyor belt. In some instances, the extracted material flows in a countercurrent manner against the direction of travel of the refined cane material. Extraction in such diffusers is by gravity combined, in some cases, with mechanical pressing.
The term xe2x80x9cdiffusionxe2x80x9d has been applied to various apparatus and methods by which a shredded, sliced, ground or refined cane or, in some instances, bagasse, is subjected to extraction by contact with heated fluid. In a true diffusion process, the plant cell is not ruptured and advantage is taken of the property of the dissolved crystals passing through the cell wall when water or other solution is more dilute than that in the cell. Following such diffusion, all solutions in contact with cells tend to achieve equal concentrations with the fluid in the cell.
Current refining practice has been to treat the cane in such a manner that, to a large extent, the cells are ruptured, and the sugar removal more nearly resembles a washing process called lixiviation. Apparently, both diffusion and lixiviation occur at the same time, but the term xe2x80x9cdiffusionxe2x80x9d has become generically applied to such extraction processes and equipment quite apart from the fact that limited actual diffusion may be taking place.
While a great deal of attention has been paid to the construction and operation of diffusion equipment, this equipment still operates principally upon through-flow arrangements which depend upon gravitational drainage, at relatively high consistency, and therefore the efficiency and production rates are accordingly limited.
Although the diffusion process has been used, it is not believed to be practiced currently to a considerable extent, having been replaced in deference to the xe2x80x9ctandemxe2x80x9d process of running the cane between sets of high pressure rolls, such as six such sets, to crush the cane under the extreme pressure of the rolls, which may exert forces of 1,000 to 4,000 pounds per square inch. The roller crushers or xe2x80x9ctandemsxe2x80x9d as they are known in the trade require a substantial power to drive, typically with the equivalent of approximately 1,500 horsepower each, and are known to present high maintenance problems.
Substantially improved production rates and efficiency can be obtained by adapting certain processing equipment that has already been developed and used in the preparation of papermakers"" pulp. The cane is xe2x80x9crefinedxe2x80x9d or macerated in a tub type pulper known in the industry as a hydrapulper. In a hydrapulper, material to be pulped is placed in a tub in which a specially designed impeller or rotor is mounted at the tub bottom above a perforated bedplate. Heated dilution water is added as necessary to achieve a desired consistency, and the pulped material is extracted through the bedplate after a sufficient period of residence time has elapsed. The pulper, or pulpers may be made according to U.S. Pat. Nos. 4,725,007 and 4,109,872, incorporated herein by reference. The impeller or rotor is preferably a xe2x80x9cMidConxe2x80x9d rotor of Thermo Black Clawson, Inc. according to U.S. Pat. No. 4,725,007 incorporated herein by reference. The hydrapulper is highly efficient in reducing the fibrous content of the cane and breaking open the cell structure, permitting release of the sugar juices into the slurry.
The pulping takes place at relatively high consistencies (approximately 8%-10%) and uses recycled filtrate from the countercurrent washer""s formation and first stage wash zones. The pulping will continue until a major portion of the sugar content is dissolved in the liquid content, such as up to 30 minutes or more to each pulper preferably at an elevated temperature, such as about 50-75xc2x0 C., so that about 70%-99% and preferably up to 98% or more of the sugar content has been extracted into the liquid phase.
When the pulping is completed, the slurry is extracted through the perforated pulper bedplate of the final pulping stage diluted to a lower consistency, and it is applied through a headbox onto the top side of the continuously moving belt of a flat bed fordrinier type countercurrent displacement type extractor. Such an extractor employs an endless foraminous belt, (often called a xe2x80x9cwirexe2x80x9d), a headbox which delivers the suspension to the on-running or upstream end of the horizontally traveling upper run of the belt while at the relatively low consistency of about 2%-4% solids at an elevated temperature. The pulped material is thus deposited on the upstream end of the run of the belt where a mat is formed as the sugar-rich liquid drains through the belt and is withdrawn for further processing.
The belt runs downstream from the formation zone and is divided into a series of displacement zones to which liquid is supplied as from showers from above for drainage through the mat and through the belt and into receptacles below the belt. Fresh washing liquid or heated water is applied at the last of these zones at the downstream end of the belt run, and the liquid drained from the last zone is collected and delivered to the zone immediately upstream from the last zone, and these steps are repeated for each of the other zones to affect countercurrent extraction from the pulp.
The entire apparatus is enclosed in a hood. A series of receptacles are positioned below the upper run of the belt in sealed relation with the hood. Suction or vacuum is applied to these receptacles. The vacuum from below and the air pressure above augment the action of gravity in forcing the liquor to flow through the pulped mass on the wire. A particular feature of the apparatus is that of recycling the gases and vapors drawn through the belt back to the hood to increase the pressure differential across the belt. The above-described apparatus is more fully described in the patents of Ericsson, U.S. Pat. No. 4,154,644, issued May 15, 1979 and Parks et al., U.S. Pat. No. 5,367,894, issued Nov. 29, 1994, which patents are incorporated herein by reference.
Preferably, immediately downstream of the final extraction stage, the pulp mass is subjected to pressure extraction by pressing one or more rolls above the belt against rolls within the belt loop, such as shown in U.S. Pat. No. 5,367,894. This provides mechanical extraction of liquid from the mat on the belt so that the pulp or bagasse remainder exits the apparatus having given up most of its liquid content, to the range of about 40% consistency. Thus, in the pressing stage, more than 70% of the liquid remaining in the pulp is expressed therefrom and captured. If desired, lime or other agents may be added to the pulp and liquid within the countercurrent extraction equipment.
A principal advantage of the use of the combination of the hydrapulper and countercurrent extraction washer resides in the fact that the hydrapulper releases 70% or more of the sugar content of the cane into the liquid content within the hydrapulper. When this is extracted through the hydrapulper bedplate with the pulped cane and applied to the headbox of the belt type extractor, the suspension is at about 2%-4% solids. On the belt, it increases from 2%-4% up to about 15% solids before final pressing. This extraction is of high efficiency by reason of the fact that it is accomplished with a pressure differential including a negative pressure head or vacuum below the belt. This translates into a high degree of extraction coupled with a high processing rate compared to that of diffusion extractors dependent upon gravity alone for separation. Since the pulp slurry applied to the belt is at low consistency and in a thin layer (2xe2x80x3 or less in thickness), the sugar remaining in the mat after draining is much less than in conventional extraction systems where the consistency is 12%-15% and the fiber depth is measured in feet. Accordingly, there remains much less sugar to be extracted at this point, as compared to conventional systems.
A further aspect of the invention resides in the employment of a pair of pulpers arranged in tandem so that they can operate on a continuous basis as distinguished from a batch basis. Where a single pulper is used, or a pair of pulpers in parallel relation, it is necessary to operate essentially each of the pulpers for a sufficient period of time, preferably alternatively, in order to provide the necessary residence time of the chopped cane in the pulper. When pulpers are arranged in tandem, each pulper may be optimized for pulping and breaking down the cane, by controlling the size of the openings in the bedplates (extraction plates). Thus, a first pulper can receive the cane in larger pieces, and it can be optimized by using an extraction plate with relatively larger openings, while a second pulper, which receives the high consistency pulp form the first pulper, can have an extraction plate with substantially smaller openings.
In either case of the pulpers operating in tandem or by the batch, it is preferred to chop the cane initially into relatively small pieces, such as by running the cane through a conventional wood yard type log chipper, such as is used for chipping limbs from trees that have been removed by road workers, power line and telephone workers, or as used in logging operations for chipping and shredding brush and branches up to 6 to 8 inches in diameter, as an example.
The chipper and pulpers entirely eliminate the first operation in a sugar cane process, namely, the conventional roller milling operation.
The apparatus and process of this invention substantially reduces the energy, i.e., horsepower, that is presently used by the sugar industry to operate the present usual arrangements of tandem presses used to extract sugar from the cane. Since the chippers and pulpers as preferably employed by this invention have few moving parts as compared to typical tandem three roll presses, maintenance problems are reduced.
It is therefore an object of the invention to extract a sugar rich liquor from sugar cane including the steps of pulping the sugar cane in sugar filtrate or water in a tub type pulper with a rotating impeller at an elevated temperature to reduce the cane to a pulp and fracture the cells to release the sugar content into the liquid phase, extracting the contents of the pulper through an extraction bedplate, applying said extracted material to the headbox of a moving belt type displacement extractor and applying said suspension onto a foraminous belt, applying a relatively negative air pressure below said belt (and/or a positive pressure on the belt) to augment the extraction of liquid content from said pulp through the belt, leaving a mat of pulp on the belt at a substantially higher consistency, and subjecting the mat to repeated zones of separation in which liquid is applied to the mat from a region above the mat and extracting liquid at the corresponding zone below the mat.
A further object of the invention includes carrying the mat through a plurality of extraction zones in which the liquid extracted from the last of said zones is applied to preceding zones in a countercurrent manner.
Another object of the present invention is to cut the sugar cane into small pieces and apply the small pieces to a tank type pulper in which a rotor at the bottom of the pulper tank is positioned to run against a perforated bedplate, adding sugar filtrate or water at an elevated temperature to said tank while operating the pulper to achieve a consistency of solid contents to liquid of about 8%-10%, continuing to operate the pulper rotor until at least about 70% or more up to about 98% of the sugar content of the cane is dissolved in the liquid phase, and draining the pulper through the bedplate and separating the liquid phase from the non-liquid phase.
A more particular object is to provide a sugar cane pulping process in which the sugar filtrate or water is added to achieve an elevated pulp temperature. The content of the pulped cane in the pulper tank is diluted to a consistency of about 2%-4% and is applied to the headbox of a moving perforated or wire belt type displacement extractor onto an on-running end of a moving wire, of applying a negative air pressure below the wire to augment the extraction of liquid content through the wire such that the consistency of the mat of pulp formed on the wire increases from about 2%-4% at the on-running end to about 15% at the off-running end, while subjecting the mat to repeated zones of separation in which liquid is applied to the fiber mat from a region above the mat and extracted from extraction zones below the mat. The mat on the wire may then be pressed between rollers at the off-running end of the wire for a final pressure extraction of liquid from the mat leaving a bagasse mat on the wire having a consistency of about 40% to about 50% or higher.
A still further object of the invention is the use of two or more pulpers in tandem, that may be operated on a continuous basis, having progressively smaller extraction plates, for higher efficiency of rupture of the sugar containing cells of the can providing for extraction of the sugar content.
Yet another object of the invention is the provision of a pulping method with displacement extraction, in which the cane is prepared for pulping by chipping in a wood yard type log chipper.
Other objects and advantages of the invention will be apparent from the following description, the accompanying drawings and the appended claims.